Put pumpkin thyme sprigs and garlic cloves in a roasting tin to fit in snugly in a single layer
Add half of the oil and toss gently to coat well Season liberally with sat and pepper and roast in a preheated oven 200C 400F Gas Mark 6 for 30 minutes until charred and softened
Meanwhile heat remaining oil in a deep saucepan Gently fry onion celery and chili for 10 mins or until softened
Add the stock bring to a boil and simmer covered for 20 minutes Stir in the roasted pumpkin and all the pan scrapings return to boil and simmer for 5 minutes
Discard thyme sprigs then transfer the soup to a food processor or blender and puree in batches until smooth Keep warm
Place all the ingredients for the coriander pistou in a spice grinder and grind to form a smooth paste or use a mortar and pestle Spoon the soup into warmed bowls and swirl a llittle pistou in each one